The present invention relates to an image sensor to be used in an input section of apparatus such as facsimile machines. More particularly, it is directed to the improvement of the image sensor having a plurality of light-receiving elements arranged in line form with each light-receiving element with a photodiode and a blocking diode connected in series-opposition.
Conventionally proposed image sensors for use in apparatus such as facsimile machines to read images are arranged so that a photodiode and a blocking diode are connected in series-opposition to form a light-receiving element. A series of the light-receiving element are arranged in a line.
More specifically, as shown in FIGS. 6 and 7, the light-receiving element portion of the above-described image sensors has a metal electrode 2 which may be comprised of Cr, for example. The light-receiving element portion is further provided with a photoconductive layer 3 which may be made of a-Si:H (amorphous silicon hydride). A transparent electrode 4 made of, for example, ITO (indium-tin oxide), is also included in the light-receiving element portion, which further includes an insulating layer 5. The insulating layer 5 can be made of polyimide, for example, which is sequentially laminated and then patterned on a transparent substrate 1. Glass can be used for the transparent substrate 1, which is further provided to form a photodiode PD and a blocking diode BD. Lead lines 7a, 7b made of, for example, Cr are arranged through contact holes 6, 6 formed in the insulating layer 5. The photodiode PD side is provided with a light-receiving region A (shaded in FIG. 7) onto which light is irradiated, while the blocking diode side is shielded by the lead line 7b so that the light is not irradiated thereonto. A plurality (n) of such a light-receiving elements are arranged to form an array. The respective lead lines 7b on the blocking diode BD side are connected to a shift register SR as shown in FIG. 8, while the respective lead lines 7a on the photodiode PD side are grounded through a loading resistor R with an output terminal Tout arranged on the side of the photodiode.
The image sensor thus arranged reads electric charges in the following way.
The shift register SR scans each photodiode PD and sequentially applies a signal thereto. Each photodiode PD to which the signal has been applied stores electric charge. During one round of such scanning operation, light is irradiated onto each photodiode PD, discharging the electric charge in amounts commensurate with an amount of the irradiated light. Then, a reset signal (read pulse) is sequentially applied from the shift register SR, causing each photodiode to recharge the electric charge in amounts commensurate with the amount of light received, and a potential generated at the output terminal T out by the current flowing through the loading resistor R at this moment is read as a signal (see, e.g., Japanese Patent Unexamined Publication No. 62978/1983).
However, the above conventional image sensor is disadvantageous as follows.
The disadvantage will be described with reference to one bit of the image sensor shown in FIG. 9. When a read pulse is applied to the blocking diode BD for a pulse duration tr, a reset voltage V whose polarity arrangement is as shown in FIG. 9 is applied during the pulse duration tr. In contrast, when a read pulse is not applied, the condition is identical to being grounded. In other words, when a pulse is applied, an electric charge of Q=CP V is stored across a photodiode PD that is reverse-biased to the reset voltage V (where CP designates a capacitance of the photodiode PD). When the pulse is completed (the condition identical to being grounded), the electric charge Q (CP V) is divided into the CP and a CB (capacitance of the blocking diode BD) according to a capacitance ratio of the CP and CB. Therefore, a divided electric charge of CP.multidot.Q/(CP+CB) is stored in the photodiode PD, and a divided electric charge of CB.multidot.Q/(CP+CB) is stored in the blocking diode BD.
When the light is irradiated onto the photodiode PD for a charging time ta and it is supposed that a photocurrent generated at this moment is i, an electric charge equal to .DELTA.q=i.multidot.ta is generated. This electric charge is likewise divided into the capacitance CP of the photodiode PD and the capacitance CB of the blocking diode BD. As a result, an electric charge of CB.multidot.i/(CB+CP) always flows across the loading resistor R through an external circuit.
As shown in FIG. 10, output waveforms produced at the time the read pulse is applied after the electric charge has been stored is such that compared to a dark output indicated by the solid line (when no light is irradiated) a photoelectric output indicated by the broken line (when light is irradiated) always includes a noise component whose amount is varied depending on the amount of irradiated light.
Let us consider a sensor consisting of a plurality (n) of bits shown in FIG. 8 in which a common electrode is arranged on the photodiode PD side. With a storing time ta, a read pulse duration tr, and a photocurrent i, the total capacitance of the electric charge that is restored into each capacitance CB during the read pulse to one of the bits is as follows. ##EQU1## And the output charge of each bit is as follows. EQU ta (CP.multidot.i)/(CP+CB)
Thus, the detected charge is as follows. ##EQU2## As a result, the following noise component is present. ##EQU3##
For example, if ta=n tr and CP=CB, a noise component equal in amount to the electric charge to be originally detected will be present (if i.sub.l to i.sub.n are equal), thereby causing the problem of inaccurate reading.
To reduce the noise component, it is conceivable that the values can be set so that the relationships ta&gt;n tr and CP&gt;CB are satisfied, but this reduces the reading speed and adds a capacitance in parallel to each photodiode, thus complicating the process (refer to Japanese Patent Unexamined Publication No. 64968/1982).